In the description hereunder, reference is made in this invention predominantly to “water vapor” gas. The insert is provided for insertion into a respective inflow and outflow apparatus. Inflow and outflow apparatuses can be employed in the application for measuring humidity, for example. Water vapor measuring installations for aircraft are known in principle and typically have an inflow and outflow apparatus (measuring probe) which serves for receiving air from the environment of an airplane and to dispense said air back to said environment. The air that is received by the inflow and outflow apparatus is supplied to the actual measuring system by way of a supply line, the determination of the water vapor content of the air then being performed in said measuring system. The measuring system that is referred to here has a measuring chamber and is accommodated within the airplane. In the operation of the airplane, air permanently flows through the inflow and outflow apparatus and by way of the supply line into and through the measuring system having the measuring chamber, and subsequently by way of the discharge line back to the inflow and outflow apparatus, and from there is dispensed back into the environment of the airplane. The air that flows through the measuring system is also referred to as measured air.